Over the past several years, significant progress has been made in understanding of the etiology and pathogenesis of periodontal diseases. Nevertheless, the nature and contribution of the immune system to these disorders remain unclear. The application presents the hypothesis that the immune system plays a primary role to minimize and/or prevent infection. Furthermore, the application posits that immunoregulatory abnormalities contribute to the pathogenesis of and susceptibility to periodontal disease. In this regard, the preliminary investigations have demonstrated that several periodontal pathogens produce factors capable of impairing human T- and B-cell function; these include Actinobacillus actinomycetemcomitans, Fusobacterium nucleatum and Treponema denticola. The fundamental hypothesis of the planned studies is that periodontal pathogens produce immunosuppressive proteins (ISPS) that mediate local and/or systemic immunosuppression, thereby enhancing their own virulence and/or that of other opportunistic microorganisms. The current plan is to focus this investigation on the A. actinomycetemcomitans ISP, recently shown to induce human lymphocytes to irreversibly arrest in the G2 phase of the cell cycle. Moreover, the applicants have determined that this ISP is the product of the cdtB gene, one of three genes encoding the family of cytolethal distending toxins. The objectives of this application are to define the cascade of events responsible for CdtB-induced G2 arrest and to determine the relationship between structure and function of this ISP. The study is composed of four Specific Aims: 1) to determine the underlying molecular mechanism(s) responsible for the failure of CdtB-treated T-cells to de-phosphorylate and, thereby activate, the cyclin dependent kinase (cdk), cdkl; 2) to determine if a receptor is involved in the binding of CdtB and/or the holo-Cdt toxin to the surface of T-cells; 3) to determine if CdtB is part of a hetero-oligomer that forms a holo-Cdt toxin; and 4) to determine if conserved motifs that exist in all known CdtB polypeptides represent functional domains of the protein.